===============================================================================

This patch makes MAC addresses of network interfaces predictable. In
particular, it adds a small routine that computes MAC addresses of based on
a SHA1 hash of the virtual machine name and interface ID.

TECHNICAL INFORMATION:

Applies to vanilla kernel 3.9.4.

===============================================================================
--- a/arch/um/Kconfig.net
+++ b/arch/um/Kconfig.net
@@ -21,6 +21,19 @@ config UML_NET
         enable at least one of the following transport options to actually
         make use of UML networking.
 
+config UML_NET_RANDOM_MAC
+	bool "Use random MAC addresses for network interfaces"
+	default n
+	depends on UML_NET
+	help
+        Virtual network devices inside a User-Mode Linux instance must be
+        assigned a MAC (Ethernet) address. If none is specified on the UML
+        command line, one must be automatically computed. If this option is
+        enabled, a randomly generated address is used. Otherwise, if this
+        option is disabled, the address is generated from a SHA1 hash of
+        the umid of the UML instance and the interface name. The latter choice
+        is useful to make MAC addresses predictable.
+
 config UML_NET_ETHERTAP
 	bool "Ethertap transport"
 	depends on UML_NET
--- a/arch/um/drivers/net_kern.c
+++ b/arch/um/drivers/net_kern.c
@@ -25,6 +25,13 @@
 #include <net_kern.h>
 #include <net_user.h>
 
+#include <crypto/sha.h>
+#include <linux/string.h>
+#include <linux/crypto.h>
+#include <linux/err.h>
+#include <linux/scatterlist.h>
+#include "os.h"
+
 #define DRIVER_NAME "uml-netdev"
 
 static DEFINE_SPINLOCK(opened_lock);
@@ -295,11 +302,47 @@ static void uml_net_user_timer_expire(un
 #endif
 }
 
+#ifndef CONFIG_UML_NET_RANDOM_MAC
+
+/* Compute a SHA1 hash of the UML instance's id and
+ *  * an interface name. */
+static int compute_hash(const char *umid, const char *ifname, char *hash) {
+	char vmif[1024];
+	struct scatterlist sg;
+	struct crypto_hash *tfm;
+	struct hash_desc desc;
+
+	strcpy (vmif, umid);
+	strcat (vmif, ifname);
+
+	tfm = crypto_alloc_hash("sha1", 0, CRYPTO_ALG_ASYNC);
+	if (IS_ERR(tfm))
+		return 1;
+
+	desc.tfm = tfm;
+	desc.flags = 0;
+
+	sg_init_table(&sg, 1);
+	sg_set_buf(&sg, vmif, strlen(vmif));
+
+	if (crypto_hash_digest(&desc, &sg, strlen(vmif), hash)) {
+		crypto_free_hash(tfm);
+		return 1;
+	}
+
+	crypto_free_hash(tfm);
+
+	return 0;
+}
+
+#endif
+
 static void setup_etheraddr(struct net_device *dev, char *str)
 {
 	unsigned char *addr = dev->dev_addr;
 	char *end;
 	int i;
+	u8 hash[SHA1_DIGEST_SIZE];
 
 	if (str == NULL)
 		goto random;
@@ -340,9 +383,26 @@ static void setup_etheraddr(struct net_d
 	return;
 
 random:
+#ifdef CONFIG_UML_NET_RANDOM_MAC
 	printk(KERN_INFO
 	       "Choosing a random ethernet address for device %s\n", dev->name);
 	eth_hw_addr_random(dev);
+#else
+	printk(KERN_INFO
+	       "Computing a digest to use as ethernet address for device %s\n", dev->name);
+	if (compute_hash(get_umid(), dev->name, hash)) {
+		printk(KERN_WARNING
+		       "Could not compute digest to use as ethernet address for device %s. "
+		       "Using random address instead.\n", dev->name);
+		random_ether_addr(addr);
+	}
+	else {
+		for (i=0; i < 6; i++)
+			addr[i] = (hash[i] + hash[i+6]) % 0x100;
+	}
+	addr [0] &= 0xfe; /* clear multicast bit */
+	addr [0] |= 0x02; /* set local assignment bit (IEEE802) */
+#endif
 }
 
 static DEFINE_SPINLOCK(devices_lock);
